In OS X 10.2, Quartz Extreme had an important weakness- it could only perform compositing, not real low-level drawing operations. This resulted in stellar performance for tasks that involved entire windows, like a transparent terminal or running a screensaver as wallpaper, but not much of an improvement for the fundamental rendering of widgets and text. 10.3 eeked out a small speed boost by accelerating rectangle copies, which mostly helped scrolling performance, but Quartz was still largely constrained by the CPU.

In Tiger, this deficiency has been corrected. The Quartz graphics engine in the latest release of Mac OS X is capable of performing all primitive graphics operations on the GPU, theoretically delivering a large performance boost and just maybe finally being able to resize windows at a reasonable frame rate on a dual G5 (TWAJS). Unfortunately, there are two caveats that will likely prevent you, the user, from reaping the benefits of Quartz 2D Extreme upon upgrading:

The computer needs to be equipped with a very high-end GPU. On the PC side the minimum requirement would be a PS 2.0-compliant graphics card; on the Mac side this means a Radeon 9600 or higher, or a Geforce 5200 or higher. So, unless you're the type to shell out for high-end G5s or overpriced Mac aftermarket graphics cards, Quartz 2D Extreme won't be supported on your system and will use a method similar to the one used on 10.3 - of course, Quartz's CPU-based renderer has received significant optimizations in the new OS version, so all systems should still see some graphics performance improvements.

Quartz 2D Extreme is disabled by default as of Mac OS X 10.4.1. Apple was unable to completely perfect and test this new technology in time for the April 2005 ship date, and decided to be better safe than sorry. It is expected to be enabled (on supported machines, see above) in a future software update. If you have installed the developer tools, you can temporarily enable the shipped but unsupported version of Quartz 2D Extreme using the Quartz Debug utility.